The present invention relates generally to pipe bending and, in particular, to a method and apparatus for bending thin wall tubing.
Thin wall tubing has been used for automobile exhaust systems for some time and, more recently has been used in heat exchangers for gas fired heating appliances. In the case of exhaust systems, the tubing is bent into a configuration to fit within the spaces available below the vehicle. In the case of heat exchangers, typically bends greater than 120xc2x0 are used to create the serpentine shape. In either case, when bends exceed 120xc2x0, the tube is commonly xe2x80x9cmandrelxe2x80x9d bent in that the tubing is bent around a forming guide while a mandrel is pulled through the inside of the tube to keep a consistent inside diameter and to prevent it from collapsing.
More recently, a technique of bending tubing using xe2x80x9ccontrol wrinklexe2x80x9d has been used. In this method, the bend die is made with a series of grooves around the inside of the bend radius, which allows the tubing material on the inside of the radius to xe2x80x9cwrinkle upxe2x80x9d in a controlled manner. This wrinkling substantially reduces excessive stretching of the material on the outside of the bend. With this more recent technique, tubing can be bent without collapsing the walls and without the use of an internal mandrel when tubing is bent more than 120 degrees.
The main disadvantage of this bending technique is that when angles exceed 90 degrees, the tubing becomes locked onto the bend die because the wrinkles in the tube engage and then are held by the grooves formed in the die. In prior constructions, the tube is released from the bend die by splitting the die so that the upper and lower halves of the die or portions thereof move apart, i.e., along a line of movement that is parallel to the rotational axis about which the tube was bent.
Tubes intended to be used as part of heat exchangers and gas fired heating products are generally serpentine in configuration and include multiple bends. In some designs, the bends in a tube are xe2x80x9coffsetxe2x80x9d with respect to each other. In order to achieve the offset bend, the tube must be rotated about its axis prior to making the subsequent bend. In the type of bending apparatus in which the bend die is split along a plane orthogonal to the axis about which the tube is bent, on subsequent bends the tooling interferes and prevents the translational movement required to reposition the tube for the next bend. In particular, in order to release the tube from the bend die, the upper half of the die moves upwardly away from the lower half of the die. When in its upper, released position, there is not sufficient room for the bent end of the tube to rotate past the upper die segment and after the second bend or on subsequent bends to translate to the next bend position. Interference may exist between the die and the tube which prevents rotation to the next bend plane. One suggested method for resolving the interference is to provide a tube bender with a xe2x80x9chead shiftxe2x80x9d capability. This solution adds to the complexity of the bending machine and is expensive. By axially shifting the xe2x80x9cbend headxe2x80x9d of the bender, the bend die is repositioned out of the way to allow the tube to clear the tooling. These added movements not only increase complexity and cost of the machine, but also add considerable time to the bending cycle.
Bending apparatuses have also been suggested which allow xe2x80x9cwrinklexe2x80x9d bent tubes to be made which involve removing some of the grooves in the portions of the die that cause the xe2x80x9clockupxe2x80x9d with the tube resulting in uncontrolled wrinkling at the extremities of the bend. Additionally, this method does not resolve the tool interference that could occur in subsequent bends.
The present invention provides a new and improved apparatus and method for bending tubing. In particular, the invention discloses an improved method and apparatus for forming multiple bends in a tube with the-bends being located in a variety of planes. The invention is especially suitable for making tubes used for heat exchangers in gas fired appliances. However, the invention can be utilized to bend tubes for other applications.
According to the invention, the apparatus includes a bending die substantially defining a profile of a bend to be produced in tube stock. A bending arm rotatably with the bending die is operative to bend the tube stock about the die. The bending die includes at least one fixed section and one moveable section which substantially define the shape of the bend that is to be imparted to the tube stock when the die sections are in a retracted position. The moveable die section is moveable between bend forming and bend release positions along a path that is substantially parallel to the plane of the bend. The path includes both a longitudinal and a lateral component so that the moveable die section moves relative to the fixed die section in both the longitudinal and transverse directions, as the moveable die section moves from its bend forming position to its bend release position. As a result, the bent portion of the tube is released from the bending die.
When the present invention is used to perform xe2x80x9cwrinklexe2x80x9d bends, the bending apparatus includes a second moveable die section which is also moveable along a path parallel to the bend plane. The path of movement for the second die section includes at least a longitudinal component. In this embodiment, as the moveable die sections move from their tube forming position to their tube release position, at least one of the moveable die sections moves towards the other moveable die sections in order to decrease the transverse distance between the moveable sections so that the bend of a tube is released from a tube forming groove that is defined by the die sections.
When the present invention is used to perform xe2x80x9cwrinklexe2x80x9d bends, at least some of the die sections include wrinkle receiving recesses which receive excess material from the inside radius of the tube being bent.
According to a feature of the invention, a cam plate supports the die sections. The cam plate includes path defining structure such as cam slots which define the path of movement for the moveable die section or die sections (if two are utilized). The moveable die section includes movement control members such as cam followers which are guided by the slots and which are also engageable with a reciprocally moveable actuating lever that, in the preferred embodiment, is also guided by the cam plate. A fluid pressure operated actuator is preferably used to produce reciprocating movement in the actuating lever.
When the present invention is used to perform mandrel type bends, the bending die may comprise a fixed die section and a single moveable die section. In mandrel type bends, wrinkles are not formed and, therefore, the tube is not locked to the die. In an alternate embodiment which is used to perform mandrel type bends, a single die section is moveable along a path defined by a cam plate. The path of movement for the single moveable die section includes both longitudinal and transverse segments so that as the moveable die section is advanced towards its tube release position, it moves both longitudinally and transversely with respect to the fixed die section. The movement of the moveable die section causes the release of the tube from the tube forming groove defined by the die sections.